FIG. 12 is an exploded perspective view of a known ultrasonic actuator. FIG. 11 is a perspective view of a piezoelectric element contained in such an ultrasonic actuator.
A piezoelectric element 10 is supported on a case 12 by a bottom-surface supporting part 8 and, as illustrated in FIG. 11, formed with four divided electrodes 9a, 9b, 9c, and 9d. An entire-surface electrode (not shown) is entirely formed at the opposite side of the piezoelectric element 10 to the side thereof formed with the electrodes 9a through 9d. 
A wire 4a is connected to the electrode 9a using a solder 5a and to the electrode 9d using a solder 5d. A wire 4b is connected to the electrode 9b using a solder 5b and to the electrode 9c using a solder 5c. Furthermore, a wire 4g is connected to the entire-surface electrode. Voltages are applied through these wires 4a, 4b and 4g to the piezoelectric element 10.
In FIG. 12, driver elements 2 are placed on the top surface of a piezoelectric element 10, and their distal ends are in contact with a movable object 3 so as to be pressed against the movable object 3 by the bottom-surface supporting part 8. This press increases the friction between the distal ends of the driver elements 2 and the movable object 3, thereby transmitting vibration of the piezoelectric element 10 through the driver elements 2 to the movable object 3 with more reliability.
Next, a driving method for this ultrasonic actuator will be briefly described.
FIG. 3 is a displacement diagram of a first-order mode of stretching vibration (a so-called longitudinal vibration; hereinafter, referred to also as a longitudinal vibration) of the piezoelectric element 10. FIG. 4 is a displacement diagram of a second-order mode of bending vibration of the piezoelectric element 10. FIGS. 5(a) through 5(d) are conceptual diagrams for explaining the geometries of the vibrating piezoelectric element 10.
The wire 4g is connected to the ground. A sinusoidal reference voltage of a specific frequency is applied to the wire 4a, and the voltage that is 90° or −90° out of phase with the reference voltage is applied to the wire 4b. Thus, the first-order mode of stretching vibration illustrated in FIG. 3 and the second-order mode of bending vibration illustrated in FIG. 4 are induced in the piezoelectric element 10.
The respective resonance frequencies of the bending and stretching vibrations are determined by the material and shape of the piezoelectric element 10 and other factors. When these two resonance frequencies are allowed to generally coincide with each other and a voltage of a frequency close to the two generally coinciding resonance frequencies is applied to the piezoelectric element 10, the second-order mode of bending vibration and the first-order mode of stretching vibration are harmonically induced in the piezoelectric element 10. This induction causes variations in the shape of the piezoelectric element 10 as illustrated in FIGS. 5(a), 5(b), 5(c), and 5(d) in this order.
As a result, the driver elements 2 placed on the piezoelectric element 10 each produce a generally elliptical motion when viewed from the front of the paper of the drawing. More particularly, the synthesis of the bending and stretching vibrations of the piezoelectric element 10 allows the driver elements 2 to each produce an elliptical motion. This elliptical motion allows the movable object 3 supported by the driver elements 2 to move in the directions shown by the arrows A and B in FIG. 12. In this way, the piezoelectric element 10 serves as an ultrasonic actuator.
For example, Patent Document 1 has been known as information on prior art documents concerning the invention of the present application.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-94956